GEB

[Chet]

Hi Green
Thank you,
Chet

[Funfinder]

It's great to consider the GEB interaction but some clay-bag of 200mm x 150mm x 80mm is not enough.
Especially if its not shure how high this material is mineralized or how concentrated.

The weight of such bag might be 3kg while an 1x1m square of red bricks 30cm thick is ca. a half ton.
If buried 10cm below the surface of such an artificial test-field the find may create already some realistic values.

But under real conditions this factor could be even 10 times higher because the mineralizied ground is everywhere.

[greylourie]

Funfinder, did you follow greens experiments ? Have a look through the data. It seems quite interesting.

[green]

It's great to consider the GEB interaction but some clay-bag of 200mm x 150mm x 80mm is not enough.
Especially if its not shure how high this material is mineralized or how concentrated.

The weight of such bag might be 3kg while an 1x1m square of red bricks 30cm thick is ca. a half ton.
If buried 10cm below the surface of such an artificial test-field the find may create already some realistic values.

But under real conditions this factor could be even 10 times higher because the mineralizied ground is everywhere.

Thanks for the reply. Some things to think about. The clay had higher signal strength at 15mm than a US nickel at 100mm with a 200mm ID coil. Do you have an idea how much higher bad ground might be compared to a US nickel?

[green]

Charted a couple of examples. Wanted to keep GEB at 120usec but didn't have enough GEB delay range to balance the clay. The clay was balanced to less than 1% of the no GEB reading for both examples. Not sure what I learned other than if you change one setting the others might have to change a lot to balance ground. Targets were at 100mm.

[mickstv]

Here's some picture of response from typical AU mineralized soil.

The TX is 8v at about 130us into a .5 ohm 300uh coil. To get GB to cancel the first sample is 6us in width, and the second is 120us in width and I adjust the delay between the end of the first sample and start of the GB sample (approx 25us but adjustable in ns steps). The circuit has another two samples later in the receive cycle, used to cancel EF.


The first picture is no target, the second is with a 5kg sample of unsorted mineralized soil sitting on top of the coil and the third picture is showing the response to a 1" piece of coke can.

The CRO was set to 2v p-p and 20us/per div. The receive gain is 100x total.

[green]

[The TX is 8v at about 130us into a .5 ohm 300uh coil. To get GB to cancel the first sample is 6us in width, and the second is 120us in width and I adjust the delay between the end of the first sample and start of the GB sample (approx 25us but adjustable in ns steps). The circuit has another two samples later in the receive cycle, used to cancel EF.


The first picture is no target, the second is with a 5kg sample of unsorted mineralized soil sitting on top of the coil and the third picture is showing the response to a 1" piece of coke can.]

Interesting, What is delay time before the first sample? Is the 1" piece of coke can sitting on the coil? Coil diameter?

[mickstv]

Interesting, What is delay time before the first sample? Is the 1" piece of coke can sitting on the coil? Coil diameter?

Hi Green, the target sample starts at about 10us. The 1" coke can is at about 4mm (the thickness of the base plate I made) and at the middle of the coil. The coil is 11" x 5" diameter using litz wire.

[crane]

The figures below are for a 235usec pulse at 5v, 310usec off-period and 10usec wide sample taken 15usec after switch off. This is with a total pre-amp/integrator gain of 280. No GB or EFE subtractions and the coil is an 8" Minelab mono (~300uH/0.35 ohms).

1986 US 5c coin sitting on the centre of the coil... 1180 mv
A ground sample (hot but not worst case)... 2,250 mv. (5 liter ice cream container of Australian mineralised dirt and rocks sitting on coil).
A relatively flat 1/2 ounce nugget... 440 mv.
A flat 1gm nugget... 30 mv.

A 10usec wide sample taken at 120 usec gives...
1/2 ouncer... 30mv
Ground sample... 180 mv

The same width sample at 300usec gives 50 mv for the ground sample and 0v for most coins other than a silver florin...30mv, and an Oz copper penny...90mv.

[Tinkerer]

Here's some picture of response from typical AU mineralized soil.

The TX is 8v at about 130us into a .5 ohm 300uh coil. To get GB to cancel the first sample is 6us in width, and the second is 120us in width and I adjust the delay between the end of the first sample and start of the GB sample (approx 25us but adjustable in ns steps). The circuit has another two samples later in the receive cycle, used to cancel EF.


The first picture is no target, the second is with a 5kg sample of unsorted mineralized soil sitting on top of the coil and the third picture is showing the response to a 1" piece of coke can.

The CRO was set to 2v p-p and 20us/per div. The receive gain is 100x total.

Great pictures.
Looking at the scope pictures, one could estimate the TC of the aluminum can piece to be about 5.6us and the TC of the soil sample about 24-25 us.
Ball-park. Does that coincide with your calculations?

Great pictures.
Looking at the scope pictures, one could estimate the TC of the aluminum can piece to be about 5.6us and the TC of the soil sample about 24-25 us.
Ball-park. Does that coincide with your calculations?

[mickstv]

Does that coincide with your calculations?

Hi Tinkerer, I haven't done any calculations, I just experimented with delay widths etc until I got it to balance my ground sample, then out in the field it only needed minor adjustments of the GB control to keep it in balance.

[green]

Would like to thank both mickstv and crane for their replies. Trying to digest it all. Some scope traces similar to mickstv. My clay is a lot less severe.

[green]

Did some comparison with cranes data. My clay is .3 of the US nickel. Cranes clay is about 2 times the nickel so his clay is about 6.5 times as severe. The quantity might be some it. It's definitely more severe. Used the simulator to compare with cranes data. Varied the start amplitude and decay slope until I got similar numbers.

[green]

What is the correct ground slope for GEB? I usually charted about -1.40. Crane's data calculates -1.36. I used the simulator to find the slope for some GEB measurements I made and mickstv's measurement. The equipment and probably the ground effect it. Think I can say -1 probably won't work.

[green]

Was playing with GEB this mourning. I have a sock with pennies, nickels, dimes and quarters in it, about the size of a baseball. With the sock close to the coil I could rotate it a little and null the signal with the GEB times I had set to balance the clay. The best sample and delay times for one situation might be worst for another. Longer GEB times seemed to help for single coins, maybe not for a lot of them. I'm still guessing what's best.